Combined Sewer Overflow

During periods of heavy rainfall or snowmelt, the wastewater volume in a combined sewer system can exceed the capacity of the sewer system or treatment plant. For this reason, combined sewer systems are designed to overflow occasionally and discharge excess wastewater directly into nearby streams, rivers, or other bodies of water. These overflows, called combined sewer overflows (CSOs), contain not only stormwater, but also untreated human and industrial waste, toxic materials, and debris.

The Environmental Protection Agency's (EPA's) Combined Sewer Overflow Control Policy, published in 1994, is a national framework for control of CSOs through the National Pollutant Discharge Elimination System (NPDES) permitting program. The policy provides guidance on how communities with combined sewer systems can meet Clean Water Act pollution control goals in the most flexible and cost-effective manner possible.

The CSO Control Policy contains four fundamental principles to ensure that CSO controls are cost-effective and meet local environmental objectives:

Clear levels of control to meet health and environmental objects

Flexibility to consider the site-specific nature of CSOs and find the most cost-effective way to control them